Tropical Storm Rosa

Tropical Storm Rosa (see 14 videos below) Satellite images and surface observations indicate that Rosa has become an elongated trough with multiple swirls along its axis. Therefore, Rosa no longer qualifies as a tropical cyclone, and this is the last advisory issued on this system by the National Hurricane Center. The initial intensity is lowered to 25 kt assuming some decrease in winds from the ASCAT pass last night.

Although Rosa has dissipated, heavy rainfall and life-threatening flash flooding are expected to continue over portions of northwestern Mexico and the Desert Southwest during the next day or so. For more information on the rainfall threat, please see products from your local NWS forecast office and Storm Summary products issued by the Weather Prediction Center.

Tropical Storm Rosa satellite imagery continues to show a lack of convection near the center of Rosa this morning. The remaining convection is located well northeast of the center, and is spreading inland over northern Mexico and portions of the Desert Southwest. A couple of partial ASCAT passes revealed an area of 25-30 kt winds over the central Gulf of California, and this is the basis for reducing the initial intensity to 30 kt. The data also suggest that the circulation has become elongated and that a new center may be forming near the northeastern coast of the Baja California peninsula.

The initial motion estimate is 030/9 kt. Rosa is expected to move northeastward between a deep-layer ridge over northern Mexico and a mid- to upper-level trough over the northeastern Pacific. These steering features should bring the center of Rosa across the northern Gulf of California and into northern Mexico this afternoon. Southwesterly shear and land interaction should cause the cyclone to continue to weaken today, and Rosa is expected to dissipate over northern Mexico or southwestern Arizona by tonight.

Although Rosa is forecast to weaken, the system is still expected to bring heavy rainfall and life-threatening flash flooding to portions of northwestern Mexico and the Desert Southwest during the next day or so.

Key Messages:

1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in those areas, please see products from the Weather Prediction Center and your local NWS forecast office.

Tropical Storm Rosa has no deep convection near its center, but there are plenty of rain-bearing clouds over areas well to the north and northeast of the center, including the northern Baja California peninsula, Sonora, and portions of the southwestern United States. The scatterometer recently missed Rosa, but assuming a steady spin down since the last overpass 12 hours ago, the advisory intensity is set at 35 kt. Continued weakening, due to very strong vertical shear, cool waters, and then interaction with land, should reduce Rosa to a tropical depression very soon. Rosa is likely to degenerate into a remnant low over the southwestern United States tomorrow.

The cyclone continues moving north-northeastward, or 030/9 kt. The track forecast philosophy is the same as in the previous advisory package. The flow ahead of a broad mid-level trough should advect Rosa, or its remnant, north-northeastward until dissipation. The official track forecast continues to follow the multi-model consensus closely.

Key Messages:

1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office.

2. Tropical storm conditions are affecting portions of the central and northern Baja California peninsula, especially over higher elevations. These conditions could spread into the northern Gulf of California later tonight. Interests in those locations should monitor the progress of Rosa.

Most of Tropical Storm Rosa’s deep convection has been sheared northeastward over portions of the Baja California peninsula, Sonora, and the southwestern United States. Locally heavy rains are already occurring over those areas. Assuming a gradual weakening of the cyclone since the last scatterometer overpass, and blending Dvorak intensity estimates from TAFB and SAB, gives a current intensity estimate of 40 kt. Since the cyclone will continue to be influenced by strong shear, cooler waters, and then interact with land, continued weakening is likely over the next day or so. Rosa is expected to degenerate into a remnant low by the time it reaches the southwestern United States on Tuesday.

The system is moving north-northeastward, or 030/10 kt. Rosa should continue to be steered on that heading, to the east of a broad mid-level trough off the California coast. Some increase in forward speed is expected after Rosa’s remnants move into the Desert Southwest. The official track forecast remains very close to the multi-model consensus.

Key Messages:

1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office.

2. Tropical storm conditions are affecting portions of the central and northern Baja California peninsula, especially over higher elevations. These conditions could spread into the northern Gulf of California later tonight. Interests in those locations should monitor the progress of Rosa.

Tropical Storm Rosa– Convection has been steadily decreasing due to the combination of southwesterly shear of about 30 kt and cool SSTs of less than 24 deg C. As a result, only a small patch of thunderstorms remains in the northeastern quadrant of Rosa’s circulation. ASCAT passes around 0414Z and 0516Z only indicated wind speeds of 41-42 kt in the eastern semicircle. Assuming that some undersampling is occurring, the intensity has only been decreased to 45 kt for this advisory. Additional weakening is expected due to a further increase in the vertical wind shear and decreasing ocean temperatures beneath the cyclone. The official intensity forecast calls for Rosa to be near 35 kt at the time of landfall later today, but it is possible that more weakening could occur than currently indicated. Regardless, strong tropical-storm-force wind gusts will be widespread across the higher elevations of the northern Baja California peninsula. Rosa is expected to become remnant low before the cyclone reaches Arizona due to the rugged terrain of Baja and northwestern Mexico, and the continued strong wind shear.

Rosa is moving northeastward now, or 035/10 kt. A northeastward to north-northeastward motion is expected until and after landfall, with some acceleration beginning later tonight. The latest NHC model guidance remains tightly clustered, and the official forecast track lies close to the previous track and an average of the TVCE, HCCA, and FSSE multi-model consensus aids.

Key Messages:

1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office.

2. Tropical storm conditions are expected over portions of the central and northern Baja California peninsula later today, especially over higher elevations. These conditions could spread into the northern Gulf of California later tonight. Interests in those locations should monitor the progress of Rosa.

Tropical Storm Rosa – An Air Force Reserve Unit Hurricane Hunter aircraft recently investigated Rosa, and found peak 700-mb flight-level winds that still supported minimal hurricane intensity. However, the SFMR-observed surface winds were somewhat lower, which is not surprising since the system has minimal deep convection and is over 24-25 deg SSTs. Given that the central pressure is still fairly low, 982 mb, Rosa is being maintained, perhaps generously, as a hurricane for this advisory. With increasing shear and cooler waters ahead for the system, weakening seems inevitable. The official intensity forecast is similar to the latest HWRF model prediction, and shows Rosa weakening to a depression by the time it reaches the southwest United States.

The initial motion is north-northeastward, or 025/10 kt. The track forecast scenario remains essentially unchanged from the previous couple of advisory packages. The flow on the eastern side of a large mid-level trough approaching from the west should steer Rosa on a north-northeastward track for the next couple of days. The official track forecast is very close to the latest dynamical model consensus. The wind radii have been adjusted slightly based on data from a recent ASCAT scatterometer overpass.

Key Messages:

1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office.

2. Tropical storm conditions are expected over portions of the central and northern Baja California peninsula on Monday, possibly spreading to the northern Gulf of California Monday night. Interests in those locations should monitor the progress of Rosa.

At 1100 AM PDT (1800 UTC), the center of Hurricane Rosa was located near latitude 24.7 North, longitude 118.4 West. Rosa is moving toward the north-northeast near 13 mph (20 km/h). This general motion is expected to continue through Tuesday. On the forecast track, the center of Rosa will approach the central and northern Baja California peninsula on Monday. Rosa’s remnants will then move across the Desert Southwest on Tuesday.

Maximum sustained winds are near 75 mph (120 km/h) with higher gusts. Weakening is forecast during the next couple of days, and Rosa is expected to become a tropical storm by this evening.

Hurricane-force winds extend outward up to 30 miles (45 km) from the center and tropical-storm-force winds extend outward up to 150 miles (240 km).

RAINFALL: Over the next few days, Rosa is expected to produce the following total rainfall accumulations: Baja California and northwestern Sonora: 3 to 6 inches, isolated 10 inches. The Mogollon Rim of Arizona: 2 to 4 inches, isolated 6 inches. Rest of the Desert Southwest, Central Rockies, and Great Basin: 1 to 2 inches, isolated 4 inches. These rainfall amounts would produce life-threatening flash flooding and dangerous debris flows in the deserts, as well as landslides in mountainous terrain.

WIND: Tropical storm conditions are expected to first reach the coast within the warning area by Monday morning. Tropical storm conditions are possible within the watch area by late Monday.

SURF: Swells generated by Rosa will affect portions of the coasts of southwestern Mexico, most of the west coast of the Baja California peninsula, and southern California through Tuesday. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office.

NWS National Hurricane Center Miami FL 800 AM PDT Sun Sep 30 2018

Hurricane Rosa continues to be affected by increasing southwesterly shear and cooler waters. Most of the deep convection has been eroded over the southern semicircle of the circulation. The current intensity estimate is reduced to 65 kt in agreement with ADT estimates from UW/CIMSS. An Air Force Reserve Unit Hurricane Hunter aircraft will investigate Rosa later today to provide a better estimate of the intensity of the system. Since the vertical shear is predicted to continue to increase and SSTs should cool to below 23 deg C by Monday, additional weakening is likely before Rosa reaches the Baja California peninsula. The official intensity forecast is near the high end of the numerical guidance.

The hurricane is moving just east of north, or around 010/11 kt. The track forecast reasoning is unchanged from the previous advisory package. Rosa is likely to be steered north-northeastward in the flow ahead of a large mid-level trough approaching from the west. This should take the center of the cyclone across Baja California in the Tropical Storm Warning area by Monday night. Rosa or it’s remnant should move over the Desert Southwest on Tuesday. The official track forecast is similar to the previous one and roughly in the middle of the dynamical guidance.

Key Messages:

1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office.

2. Tropical storm conditions are expected over portions of the central and northern Baja California peninsula on Monday, possibly spreading to the northern Gulf of California Monday night. Interests in those locations should monitor the progress of Rosa.

Hurricane Rosa –Infrared and microwave satellite images indicate a continued erosion of Rosa’s inner-core structure since the previous advisory due to southwesterly vertical wind shear near 20 kt along with colder water beneath the hurricane and entrainment of drier mid-level air in the southwestern quadrant. The intensity was assessed as 80 kt at 0600 UTC based on the high-end estimates from TAFB and UW-CIMSS ADT and SATCON, but since then the rapid erosion of the eye feature and strong northeastward tilt to the vortex column noted in microwave imagery suggests a lower estimate of 75 kt for the advisory intensity.

This initial motion estimate is 005/10 kt. Rosa is forecast to continue moving northward around the western edge of a deep-layer ridge for the next 24 h or so, followed by a turn toward the north-northeast on Tuesday as a mid-/upper-level trough approaches from the west. As the low- and upper-level circulations continue to decouple, Rosa should essentially maintain its current forward speed until landfall occurs in 36-48 hours due to the cyclone not being influenced by the faster deep-layer steering flow. The new NHC track forecast is a little slower than the previous advisory track, and closely follows the consensus models HCCA and IVCN. A 72-hour forecast position continues to be provided for continuity purposes, but Rosa’s surface circulation is likely to dissipate before that time over northwestern Mexico or southern Arizona, with the mid-level remnants continuing northward across the Desert Southwest and Intermountain West.

Rosa is now moving over waters colder than 25 deg C, with colder water near 22 deg C ahead of the cyclone just prior to landfall. The combination of increasing wind shear, cooler waters and drier and more stable air being entrained from the west should result in steady or even rapid weakening of the cyclone until landfall occurs. The official forecast follows the sharp weakening trend indciated in the previous advisory, which is supported by the latest intensity guidance. Rosa is expected to devolve into an exposed low-level center with the associated deep convection being sheared off to its north and northeast by the time it is nearing the Baja California coast on Monday. However, it will take some time for the circulation to spin down, and Rosa is still expected to bring tropical-storm-force winds to portions of Baja California in 36-48 hours.

Key Messages:

1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office.

2. Tropical storm conditions are expected over portions of the central and northern Baja California peninsula on Monday, possibly spreading to the northern Gulf of California Monday night. Interests in those locations should monitor the progress of Rosa.

Hurricane Rosa’s strengthening trend seen earlier has at least slowed, as Rosa has shown little change in organization since the last advisory. The hurricane continues to have a well-defined 30-40 n mi wide eye inside a central dense overcast with cloud tops as cold as -75C. The various satellite intensity estimates have changed little, so the initial intensity remains 90 kt. The hurricane has good to excellent cirrus outflow in the northeastern semicircle, and the outflow has recently improved in the southwestern semicircle.

Rosa is now crossing the 26C isotherm, so additional significant strengthening appears unlikely. After 12 h, the combination of decreasing sea surface temperatures and increasing shear should cause steady to rapid weakening, and the intensity forecast continues to call for the cyclone to weaken below hurricane strength before the center reaches the Baja California peninsula in just over 48 h. After landfall, the surface circulation is forecast to dissipate near the 72-h point in agreement with all of the dynamical models. However, the mid-level circulation and the associated rainfall will continue moving across the southwestern United States after the surface circulation dissipates.

The initial motion is 350/10. During the forecast period, the hurricane will recurve into the westerlies between a deep-layer ridge over northern Mexico and a large mid- to upper-level trough over California and the adjacent Pacific. This should result in a continued northward motion through tonight, followed by a turn toward the north-northeast on Sunday. The new forecast track is a little to the left of the previous track for the first 12 h, but otherwise is changed little from the previous track. On the forecast track, the center of Rosa or its remnants will move near or over the central and northern portions the Baja California peninsula on Monday and Monday night, and then move into the southwestern United States on Tuesday.

Key Messages:

1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Mexico, and the Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office.

2. Rosa could also bring tropical storm conditions to portions of the central and northern Baja California peninsula starting on Monday. Interests in those locations should monitor the progress of Rosa.

Hurricane Rosa’s satellite appearance has degraded significantly since the previous advisory with an eye no longer evident in infrared imagery. However, a well-defined low-level eye is evident in recent passive microwave imagery. A comparison of the two data sources indicate that the upper-level circulation is tilted to the east of the low-level eye by 12-18 nmi due to westerly shear of about 15 kt as assessed by UW-CIMSS. The initial intensity of 85 kt is based on a blend of satellite intensity estimates form TAFB, SAB, and UW-CIMSS ADT and SATCON.

Rosa is moving northward now and the initial motion estimate is 350/08 kt. During the next 72 hours, the hurricane is forecast to move around the western periphery of a deep-layer ridge that extends from the Gulf of Mexico westward to Baja California, resulting in a northward motion today, and a turn toward the north-northeast at a faster forward speed on Monday and Tuesday as a mid-latitude trough approaches from the west. The latest NHC model guidance is tightly packed about the previous forecast track, so no significant changes were required. The new NHC track forecast lies close to a blend of the consensus models HCCA, FSSE, and IVCN.

Rosa’s intensity has decreased 40 kt during the past 24 hours, with the bulk of the weakening having occurred during the past 18 hours, and additional weakening is expected due to the cyclone moving over cooler water and into a regime of increasing southwesterly vertical wind shear of more than 30 kt by 48 hours. Although Rosa is currently located over 28 deg C SSTs, the depth of the warm water is quite shallow, so cold upwelling occurring beneath the hurricane will act to hasten the weakening process today. Rosa is forecast to approach Baja California as a tropical storm, and then quickly degenerate into a tropical and a remnant low as the cyclone moves across the mountainous terrain of northern Baja California and northwestern mainland Mexico. The official intensity forecast closely follows the intensity consensus models IVCN and HCCA.

Key Messages:

1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Mexico, and the Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office.

2. Rosa could also bring tropical storm conditions to portions of the central and northern Baja California peninsula in a couple of days. Interests in those locations should monitor the progress of Rosa.

Hurricane Rosa’s appearance has rapidly deteriorated since this morning. There is still evidence of an ongoing eyewall replacement cycle, and an apparent moat region has occasionally been apparent in conventional satellite data during the past several hours. The slow motion of the hurricane since last night could also be causing upwelling that would lead to further weakening. Satellite intensity estimates from all agencies have dropped substantially accordingly, and the initial intensity has been lowered to 105 kt, based on a blend of Final-T and CI numbers from TAFB and SAB.

Rosa is still moving northwestward, with an initial motion of 325/5 kt. The model spread has increased a little, with the GFS and its associated regional models showing a slightly right (southeast) track, compared to earlier cycles, while the ECMWF has shifted an equal amount to the left (northwest). The consensus models have not changed much as a result of these offsetting model trends, so almost no change was made to the NHC track forecast. Rosa is still expected to turn northward, and then north-northeastward ahead of a mid- to upper-level trough approaching from the northwest. The cyclone should then accelerate a little as it approaches the northern coast of the Baja California peninsula early next week.

At this point, the structure of Rosa has degraded to the point that substantial restrengthening appears unlikely. Since the hurricane still has about 24-36 hours before it reaches much cooler SSTs, only gradual weakening is forecast, though most of the intensity guidance shows more rapid weakening than currently indicated. After that time, a more rapid rate of weakening could occur due to cooler SSTs and an increase in wind shear associated with an upper-level trough approaching from the west.

By early next week, Rosa is forecast to move inland over northern Baja California, and its surface circulation will likely dissipate shortly thereafter. However, moisture associated with Rosa is expected to spread northeastward through parts of the southwest United States. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office.

Key Messages:

1. Rosa could bring tropical storm conditions to portions of the central and northern Baja California peninsula in a few days. Interests in those locations should monitor the progress of Rosa.

2. Heavy rainfall associated with Rosa or its remnants is expected to affect parts of the southwest United States by early next week, which could cause flooding in this region. For more information about potential rainfall and flooding, please see products from the Weather Prediction Center and your local NWS forecast office.

Hurricane Rosa Corrected typo in second paragraph. Cloud tops surrounding the distinct eye of Rosa have warmed considerably over the past few hours. A recent SSMI overpass showed a distinct double eyewall structure, and this is likely the reason for the degradation of Rosa’s appearance. Dvorak intensity estimates from TAFB and SAB have decreased as a result, and the initial intensity has been therefore decreased to 120 kt. Given the continued warming of cloud tops since 1200 UTC, it is possible that Rosa has weakened a little more than this.

Since an eyewall replacement cycle is ongoing, Rosa is no longer expected to intensify. In the short term, some additional weakening is likely, typical of major hurricanes undergoing eyewall replacements. If this process completes within the next 24 hours, there will be some opportunity for the hurricane to re-intensify while the shear is low and the hurricane remains over warm SSTs. However, steady weakening is anticipated beginning Saturday night since the cyclone is forecast to cross over much cooler SSTs. By 96 h, a weaker Rosa will likely approach the northern west coast of the Baja California peninsula and the resulting interaction with land will likely cause the cyclone to dissipate. It is worth noting that a farther northwest track would likely result in Rosa weakening faster than currently indicated, while a farther southeast track could allow Rosa to maintain a slightly stronger circulation as it crosses the northern Gulf of California.

Rosa has begun its long-anticipated turn toward the northwest, and the initial motion estimate is 315/6 kt. The track models remain in good agreement that Rosa will gradually turn toward the north, and then north-northeast over the next few days ahead of an approaching mid- to upper-level trough to the northwest. No significant changes were made to the official track forecast, but it has been adjusted slightly to the left (or northwest) at most times, in line with the latest consensus guidance.

Even though the NHC forecast calls for the surface circulation of Rosa to dissipate after making landfall along the northern Baja California coast, its associated moisture is expected to spread northeastward through parts of the southwest United States. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office.

Although enhanced BD-curve infrared imagery shows that Hurricane Rosa’s 20 n mi wide eye has continued to warm (+16C) this morning, the southwest quadrant of the eyewall appears to have weakened, or collapsed, and the inner ring cloud tops have warmed considerably. This change in the cloud pattern maybe the early stage of an eyewall replacement cycle (ERC). It’s certainly worth noting, however, that the satellite presentation about 3 hours ago indicated that Rosa may have reached a peak intensity of 130-135 kt which was also indicated in the ADT adjusted raw T-numbers. For this advisory, the initial intensity is set at 125 kt and is based on a compromise of the subjective and objective satellite intensity estimates.

Rosa has a small window of opportunity to complete its ERC and restrengthen before it encounters decreasing oceanic temperatures, and increasing southwesterly shear. These inhibiting factors, along with Rosa moving into a more stable air mass, should cause a downward intensity trend by early next week, and for the cyclone to weaken to a tropical storm by Tuesday, and a tropical depression as it quickly moves northeastward over the southwestern U.S. The intensity forecast is similar to my predecessor’s and is close to the IVCN consensus through 48 hours, then quite similar to the NOAA-HCCA intensity model beyond that forecast period.

The initial motion is estimated to be westward, or 270/6 kt. A mid-level ridge over the southwestern United States is forecast to weaken over the next couple of days in response to an approaching mid-level shortwave trough from the northwest. This growing weakness in the ridge should influence Rosa to gradually turn northward through mid-period, and afterward, turn northeastward within the strong mid- to upper tropospheric southwesterly flow produced by the aforementioned trough. The NHC track forecast was adjusted ever so slightly to the left of the previous one to light more closely to the TVCN and HCCA multi-model guidance.

Hurricane Rosais becoming better organized. Conventional satellite imagery show a large area of cold cloud tops near the center, and there is a hint of an eye in the first-light visible images. Microwave imagery indicates that the eye structure underneath the overcast has become better defined, with less evidence of dry air entrainment than seen yesterday. The initial intensity has been increased to 90 kt in best agreement with the subjective Dvorak estimate from TAFB and the CIMSS ADT technique. The hurricane currently has good outflow in all directions.

Conditions appear generally favorable for continued strengthening for the next 24-36 hr, and the new intensity forecast now makes Rosa a major hurricane in 12 h. It should be noted that if the current strengthening is the start of the previously anticipated rapid intensification Rosa could get stronger than the current forecast, which shows a peak intensity of 105 kt near the upper edge of the intensity guidance. After 36 h, the forecast track takes the cyclone over decreasing sea surface temperatures, and by the end of the forecast period it is expected to encounter strong southwesterly vertical shear. This combination should cause significant weakening, and Rosa is expected to be a tropical storm as it approaches the Baja California peninsula near the end of the forecast period.

The initial motion is a somewhat uncertain 270/10. For the next 12-24 h, the hurricane should move generally westward on the south side of the subtropical ridge over the Pacific west of northern Mexico and a mid-latitude ridge over California. Subsequently, Rosa should turn northwestward and northward through a break in the ridge caused by a large mid-latitude trough moving eastward through the northeastern Pacific. By 96-120 h, Rosa should recurve northeastward into the westerlies on the eastern side of the trough and move in the general direction of northwestern Mexico and the northern Baja California peninsula. There remains some spread in the guidance forward speed after recurvature, with the GFS being faster than the ECMWF. The new intensity forecast is a blend of these extremes in forward speed at the 96 and 120 h points. Overall, the guidance envelope has shifted a little west since the previous advisory, and the new track is also shifted a little westward.

Hurricane Rosa continues to strengthen this afternoon. Improved deep convective banding features with -83C cloud tops are evident in both visible and microwave images, particularly in the east semicircle. The initial intensity is bumped up to 70 kt and is a compromise of the subjective and objective satellite intensity estimates.

It still appears likely that Rosa will continue to quickly strengthen through this evening while it moves over warm oceanic temperatures and in a high moisture and low shear atmospheric environment. The official forecast is based on a blend of the IVCN and NOAA-HCCA intensity consensus models, and shows strengthening, at a slower pace, after the RI period. Beyond 48 hours, steady weakening is forecast due to decreasing sea surface temperatures, increasing southwesterly shear, and a high statically stable surrounding environment.

Rosa is currently moving west-northwestward, or 285/9 kt, within the mid-level easterly steering flow produced by a mid-level ridge extending from northern Mexico westward over the eastern Pacific. At around day 3, Rosa is expect to move northwestward and then northward day 4 in response to a mid-latitude trough approaching from the northwest. The along track spread beyond day 3 continues to be an issue with the GFS global and HWRF hurricane model indicating a much more faster north to north-northeast motion than the slower European model cluster. In this scenario, the NHC forecast will remain near the better performing various multi-model consensus models.

Hurricane Rosa is undergoing rapid intensification. Rosa’s inner core has quickly consolidated, indicated by a 1134 UTC SSMI/S image that showed a solid mid-level ring with a strong well-developed banding feature in the south semicircle. The lower frequency image from the pass indicated a small break in the north portion, probably due to the drier air and northeasterly shear that had been inhibiting the cyclone’s cloud pattern last night. Based on the conventional and microwave imagery presentation, the initial intensity is raised to 65 kt.

Rosa should continue on this fast strengthening trend during the next 12 hours or so. Afterward, further intensification, at a slower pace, is expected during the next couple of days. Through the remaining portion of the forecast, gradual weakening is expected as a result of decreasing sea surface temperatures, increasing southwesterly vertical shear, and an invading stable, more drier, marine layer air mass.

Hurricane Rosa Distances 0500 Hours September 26 2018

The initial motion is estimated to be west-northwestward, or 290/9 kt, within the easterly mid-tropospheric flow produced by a ridge to the north of the hurricane. Around mid-period, the large-scale models show a weakness developing the in the aforementioned ridge in response to a mid-latitude trough approaching the southwest U.S. and Baja California peninsula. As a result of this synoptic pattern change, Rose should move northwestward Saturday and then north-northwestward Sunday. The large along and cross-track spread recognized in the guidance 24 hours ago has decreased significantly, increasing the forecast confidence some beyond 48 hours. The NHC forecast is fairly close to the previous one, and is based primarily on the multi-model consensus.

Satellite imagery indicates that Rosa is still strengthening. The tropical storm has impressive banding features that wrap cyclonically from the northwest to northeast quadrant, though a dry slot has recently been observed infringing on the storm’s developing inner-core. The most recent Dvorak intensity estimates from TAFB and SAB are 55 kt, so the initial intensity has been raised to that value.

All of the intensity guidance suggests that rapid intensification (RI) is either already underway or imminent. SSMIS imagery from around 2300 UTC appeared to confirm the onset of RI with the development of a small low-level eye-like feature that has been associated with rapid intensification in past tropical cyclones. Although more recent imagery indicates this feature may have been disrupted by the aforementioned dry slot, the models suggest that this disruption may not last long. The NHC forecast therefore continues to show RI, and now calls for Rosa to become a major hurricane in about 36 h. Beyond that time, the hurricane is expected to remain in a fairly favorable environment through Friday, however, eyewall replacement cycles could limit the extent to which Rosa intensifies once it nears major hurricane strength. By the weekend, cooler SSTs and a drier surrounding environment should cause Rosa to begin weakening. The NHC intensity forecast is similar to the previous advisory, and is generally close to HCCA, especially beyond 48 h.

Rosa is moving steadily west-northwestward, or 285/8 kt. A mid- level ridge to the north of Rosa should keep the cyclone on the same general heading for the next several days. There is still some large discrepancies between the global models on the strength of the ridge for the next few days, resulting in differences in the forward speed of Rosa during that time. By the end of the week, a large mid-latitude trough is expected to create a weakness in the ridge, allowing Rosa to turn northwestward, and possibly northward. The NHC track forecast will continue to follow the TVCE and HCCA consensus aids for this advisory.

The cloud pattern of the tropical cyclone has become better organized, with developing convective banding features. Dvorak intensity estimates are 35 kt and 45 kt from SAB and TAFB, respectively, and the current intensity estimate is set at 40 kt. Thus the system is being upgraded to a tropical storm. Conditions favor continued strengthening, with Rosa likely to remain in an environment of warm waters, low shear, and a very moist mid-level air mass for the next several days. The official intensity forecast is a little higher than the previous one, but below some of the objective guidance. By late in the forecast period, a gradual weakening trend should commence as the system begins to move over slightly cooler waters.

The storm is moving west-northwestward, or 285/8 kt. A mid-level ridge is forecast by the global models to remain in place to the north of Rosa through the middle part of the forecast period. This would likely maintain a generally west-northwestward motion for the next 72 hours or so. By days 4-5, the ridge is predicted to weaken which should result in a gradual turn toward the northwest and north-northwest. The official track forecast is similar to that from the previous advisory and is close to the corrected dynamical model consensus, HCCA.

The low pressure system that NHC has been monitoring off the coast of southwestern Mexico has now become a tropical depression, the twentieth of the eastern North Pacific 2018 season. ASCAT data from around 0400 UTC indicated that the system had a well-defined center and maximum winds in the 25-30 kt range. In addition, satellite images show deep convection increasing in coverage and intensity near the center. The initial wind speed is set at 30 kt, based on the ASCAT data, but this is a little below the latest Dvorak estimate from TAFB.

Since the depression has only recently formed, the initial motion is an uncertain 280/7 kt. Mid-level ridging to the north of the system should keep the depression on a westward to west- northwestward path at about the same forward speed for the next few days. Thereafter, the models show the ridge breaking down due to a large cut off deep-layer low moving toward the west coast of the United States. In response, the cyclone is expected to turn to the northwest this weekend. The models agree on this overall scenario, but there is a fair amount of spread by the end of the forecast period. The NHC track forecast lies near the middle of the guidance envelope and close to the best performing models, the consensus aids.

The environmental conditions appear favorable for the depression to strengthen. SSTs are expected to remain quite warm beneath the cyclone for the next several days while vertical wind shear values are around 10 kt or less. These conditions combined with a moist airmass should allow for at least steady strengthening during the next 3 to 4 days. Some weakening is possible by the end of the forecast period due to slightly cooler SSTs and an increase in shear. The NHC intensity forecast shows a slower rate of strengthening than the guidance, since the system is still in the developing stage, but predicts peak winds near the IVCN consensus model.

For the eastern North Pacific…east of 140 degrees west longitude: The National Hurricane Center is issuing advisories on newly formed Tropical Depression Nineteen-E located over the southern Gulf of California.

(see Spanish video below)

1. An elongated area of low pressure located about 850 miles southwest of the southern tip of the Baja California peninsula has become less organized today. Environmental conditions could be conducive for some gradual development of this system during the next few days while it moves slowly west-northwestward or northwestward. * Formation chance through 48 hours…low…10 percent. * Formation chance through 5 days…low…30 percent.

2. An area of disturbed weather is expected to form over the weekend a few hundred miles south of the Gulf of Tehuantepec. Some slow development of this system is possible early next week while the system moves nearly parallel to the southwestern coast of Mexico. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…low...20 percent.

Satellite images show that an area of low pressure has developed over the Gulf of California overnight within an inverted trough. This is also supported by 24-hour pressure falls of 3.7 mb at Loreto just west of its center at 1300 UTC. Microwave imagery shows curved convective banding features to the east and north of the center, suggesting organized convection.

Therefore the system is being declared a tropical depression with 25-kt maximum sustained winds. Infrared imagery shows minimal vertical wind shear over the system, with a slight restriction to the outflow on its western side, which is confirmed by recent SHIPS output. However, the system has only 12 hours or less over water, and it is expected to move inland without significant strengthening.

An upper-level trough moving into the western United States is expected to steer the depression north or north-northeast across the Gulf of California into northwest Mexico tonight, which is well advertised by the track guidance. The depression is expected to dissipate in the 24-36 hour time frame due to steep terrain.

The main impact with the system is expected to be heavy rainfall, with 5-10 inch areal average amounts and local amounts to 15 inches leading to life-threatening flash flooding and landslides near the track of the depression.

1. A broad and elongated area of low pressure extending from near the southern tip of the Baja California peninsula southwestward for a few hundred miles continues to produce widespread but disorganized showers and thunderstorms. Although environmental conditions appear conducive for tropical cyclone formation, the large size of the system and land interaction is expected to limit the development potential of this disturbance. However, this system could still become a tropical depression during the next day or two while it moves near Baja California Sur and the Gulf of California. Regardless of development, this disturbance will likely produce very heavy rainfall over Baja California Sur and other parts of northwestern mainland Mexico over the next several days. Interests in Baja California Sur and northwestern Mexico should monitor the progress of this disturbance. * Formation chance through 48 hours…medium…40 percent. * Formation chance through 5 days…medium...40 percent.

2. Another area of low pressure is located about 800 miles southwest of the southern tip of the Baja California peninsula. Environmental conditions are expected to be marginally conducive for some gradual development of this system over the next several days while it moves slowly west-northwestward or northwestward. * Formation chance through 48 hours…low…10 percent. * Formation chance through 5 days…low...20 percent.

NWS National Hurricane Center Miami FL 500 AM PDT Mon Sep 17 2018

For the eastern North Pacific…east of 140 degrees west longitude:

1. A broad and elongated area of low pressure located a few hundred miles south of the southern tip of the Baja California peninsula continues to produce widespread but disorganized showers and thunderstorms. Although environmental conditions appear conducive for tropical cyclone formation, the large size of the system suggests that any development should be slow to occur.

This system will likely become a tropical depression later this week while it moves generally northward at about 10 mph. Interests in Baja California Sur should monitor the progress of this disturbance.

1. A large area of disturbed weather centered several hundred miles southwest of Mexico is associated with a trough of low pressure. Environmental conditions appear conducive for gradual development of this system, and a tropical depression is likely to form by the middle of next week while it moves generally northwestward at about 10 mph. Interests in Baja California Sur should monitor the progress of this disturbance. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…high...80 percent.

NWS National Hurricane Center Miami FL 500 PM PDT Sat Sep 15 2018

For the eastern North Pacific…east of 140 degrees west longitude:

1. A large area of disturbed weather centered several hundred miles southwest of Mexico is associated with a trough of low pressure. Environmental conditions appear conducive for gradual development of this system, and a tropical depression is likely to form by the middle of next week while it moves generally northwestward at about 10 mph. Interests in Baja California Sur should monitor the progress of this disturbance. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…high…80 percent.

NWS National Hurricane Center Miami FL 500 AM PDT Sat Sep 15 2018

For the eastern North Pacific…east of 140 degrees west longitude:

A large area of disturbed weather centered several hundred miles south of the southern coast of Mexico is associated with a trough of low pressure. This system is gradually becoming better organized, and environmental conditions appear conducive for further development. A tropical depression is likely to form by Tuesday while the system moves generally west-northwestward at about 10 mph. Interests in Baja California Sur should monitor the progress of this disturbance. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…high…80 percent.

NWS National Hurricane Center Miami FL 1100 AM PDT Fri Sep 14 2018

For the eastern North Pacific…east of 140 degrees west longitude:

1. A large area of disturbed weather is centered several hundred miles south of the southern coast of Mexico. Environmental conditions appear conducive for gradual development, and a tropical depression could form early next week while the system moves generally westward to west-northwestward at 10 to 15 mph south of Mexico. * Formation chance through 48 hours…low…10 percent. * Formation chance through 5 days…medium…60 percent.

If this system is named, it will be Tropical Storm Rosa.

NWS National Hurricane Center Miami FL 500 AM PDT Thu Sep 13 2018

For the eastern North Pacific…east of 140 degrees west longitude:

An area of low pressure is expected to develop well south of the southwestern coast of Mexico over the weekend. Thereafter, environmental conditions appear conducive for gradual development, and a tropical depression could form while the system moves generally westward to west-northwestward at 10 to 15 mph south of Mexico. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…medium…40 percent.

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